The invention relates to a color cathode-ray tube having an electron gun with three electron lenses, and, more particularly, to a three-lens electron gun capable of providing asymmetrically-shaped electron beams of substantially constant current density.
FIG. 1 shows a conventional rectangular color picture tube 10 having a glass envelope 11 comprising a rectangular faceplate panel 12 and a tubular neck 14 connected by a rectangular funnel 16. The panel 12 comprises a viewing faceplate 18 and a peripheral flange or sidewall 20 which is sealed to the funnel 16 by a frit seal 21. A mosaic three-color phosphor screen 22 is located on the inner surface of the faceplate 18. The screen preferably is a line screen with the phosphor lines extending substantially perpendicular to the high frequency raster line scan of the tube (normal to the plane of the FIG. 1). Alternatively, the screen could be a dot screen. A multi-apertured color selection electrode or shadow mask 24 is removably mounted, by conventional means, in predetermined spaced relation to the screen 22. An inline electron gun 26, shown schematically by dashed lines in FIG. 1, is centrally mounted within the neck 14 to generate and direct three electron beams 28 along initially coplanar beam paths through the mask 24 and toward the screen 22. One type of electron gun that is conventional is a four grid bi-potential electron gun such as that shown in FIG. 2 and described in U.S. Pat. No. 4,620,133 issued to Morrell et al. on Oct. 28, 1986, which is assigned to the assignee of the present invention and is incorporated by reference herein for the purpose of disclosure.
The tube of FIG. 1 is designed to be used with an external magnetic deflection yoke, such as yoke 30 located in the region of the funnel-to-neck junction. When activated, the yoke 30 subjects the three beams 28 to magnetic fields which cause the beams to scan horizontally and vertically in a rectangular raster over the screen 22. The initial plane of deflection (at zero deflection) is shown by the line P--P in FIG. 1 at about the middle of the yoke 30. Because of fringe fields, the zone of deflection of the tube extends axially from the yoke 30 into the region of the gun 26. For simplicity, the actual curvature of the deflected beam paths in the deflection zone is not shown in FIG. 1. The yoke 30 provides an inhomogeneous magnetic field that has a strong pin cushion-like vertical deflection magnetic field and a strong barrel-like horizontal deflection magnetic field to converge the electron beams at the peripheral part of the screen 22. When the electron beams pass through such an inhomogeneous magnetic field, the beams are subject to distortions and defocusing. As a result, at the peripheral portions of the screen 22 the shape of the electron beam spot is greatly distorted. FIG. 3 represents an electron beam spot for a single beam which is circular at the center of the screen and undergoes various types of distortions at the periphery of the screen 22. As shown in FIG. 3, the beam spot becomes horizontally elongated when deflected along the horizontal axis. The beam spot at the four corners of the screen comprises a combination of horizontally elongated portions and vertically elongated portions that form elliptically-shaped spots with halo-shaped elongations thereabout. The resolution is degraded as the electron beam is deflected, and the non-uniform focusing cannot be neglected and presents a problem which must be addressed.
The aforementioned U.S. Pat. No. 4,620,133 addresses the beam focus problem by providing an improved color imaging display system that includes a deflection yoke and an electron gun that has both an improved beam forming region, comprising a first grid, G1, a second grid, G2, a third grid, G3, and an improved main focusing lens, G3-G4, which works in conjunction with the deflection yoke and the beam forming region to provide an improved beam spot at the screen 22. FIG. 4a shows an electron beam current density contour, at the center of the screen 22, for an electron beam produced by the beam forming region and the main lens of the electron gun shown in FIG. 2. The beam current of the electron gun is 4 milliamperes. The electron beam current density contour of FIG. 4a comprises a relatively large center portion having a substantially constant beam current of about 50% of the average beam current, and peripheral portions where the beam current drops to about 5% of the average beam current and finally to about 1% of the average beam current. The beam is elliptically-shaped along the vertical axis to reduce the overfocusing action of the yoke when the beam is deflected. FIG. 4b shows the beam current density contour within the main lens, L2, that is between the G3 and G4 electrodes of FIG. 2. The electron beam at this location is horizontally elongated; however, the 50% beam current density portion is contained within the small elliptical center section of the beam which is circumscribed by the larger elliptical portions which represent the 5% and 1% beam current density profiles. FIG. 4c shows the electron beam current density contour of the electron beam deflected into the upper right hand corner of the screen. Some haloing occurs above and below the central portion of the beam. FIG. 5 depicts the paths of the electrons emerging from the beam forming region of the electron gun of FIG. 2 for various beam currents. In FIG. 5a the beam current is adjusted to 4 milliamperes and a crossover point occurs at about 2.8 to 2.9 mm (110 to 115 mils) from the cathode located at the origin. At a distance of about 5.2 mm (200 mils) from the origin, the electrons are concentrated in the center portion of the beam. This distribution of electrons produces the current density contour at the screen, shown in FIG. 4a. The effect of beam current on the location of the crossover point and on the beam current density contour is shown in FIGS. 5b and 5c. In FIG. 5b, when the beam current is decreased to 0.8 milliamperes, the crossover point is shifted to a location about 1.14 mm (45 mils) from the cathode. It is apparent that the divergence angle of the electron beam is somewhat less at an operating current of 0.8 milliamperes than at an operating current of 4.0 milliamperes (FIG. 5a). In FIG. 5c at a beam current of 0.2 milliamperes, the crossover point is located less than about 0.6 mm (25 mils) from the cathode and the beam is virtually a laminar beam.
U.S. Pat. No. 4,641,058 issued to Koshigoe et al. on Feb. 3, 1987 also discloses a four-grid bi-potential electron gun in which a prefocused astigmatic lens is formed between the second and third grids and a main astigmatic focus lens is formed between the third and fourth grids. The advantage of the patentees' two-lens structure over prior bi-potential structures is that unlike various types of prior bi-potential electron guns, such as that shown in FIG. 2, which provide an astigmatic shape to the electron beam by means of the first grid, the patentees have utilized the second and/or third grids as the first astigmatic lens. The latter structure allegedly permits the astigmatic electron beam formed by the first astigmatic lens to be compensated for in the main astigmatic focus lens to provide a substantially circular-shaped beam spot on a phosphor screen of a cathode ray tube. The structure described in the Koshigoe et al. patent is also applied to a composite lens type electron gun having six grids and three separate electron lenses such as that shown in FIG. 6. In the six grid structure of Koshigoe et al., the first (prefocus) lens, L1, is formed between the second and third grids, the third, fourth, and fifth grids constitute a sub-lens, L2, and the fifth and sixth grids constitute a main lens, L3. In this latter embodiment, the first (prefocus) lens serves as the first astigmatic lens and the main lens serves as the second astigmatic lens. The patentees claim that the deflected beam spot in this electron gun is superior to that obtained in prior art electron guns. However, in such an electron gun structure the location of the crossover point is dependent upon the beam current of the electron gun. While the first asymmetric lens, L1, is formed in the region between the second and third grid electrodes, G2 and G3 respectively, the crossover point may occur either before or after lens L1, depending upon the electron beam current. At a high beam current of about 4 milliamperes (ma), the crossover point occurs after the lens L1, i.e. closer to the G3 electrode. Thus, the asymmetric effect of lens L1 is a function of beam current. It is therefore desirable to provide an electron lens which is insensitive to the beam current, i.e., the asymmetric lens should be located beyond the crossover point, regardless of the operating beam current of the electron gun. Additionally, it is desirable to have a gun structure which provides beams of substantially constant current density, in both the horizontal and vertical directions, in the main lens.